1. Field of the Invention
The present invention relates to a power supply having a digitally implemented slew rate controller, and more particularly to a power supply having a digitally implemented slew rate controller which optimizes up and down programming speed in programmable power supplies while minimizing overshoots.
2. Description of the Related Art
Power supplies are a useful tool for performing system tests of electronic devices during manufacture of the devices and test system throughput is a critical factor in controlling manufacturing overhead. Cost of test is increasing as products become more sophisticated, requiring more complicated tests. Power supplies are a part of a minimum set of test apparatus connected during any system test and are a significant part of total system throughput.
System power supplies are required to change output state quickly to maximize system throughput. A system power supply should settle to a new final value in a minimum time, without undershooting or overshooting the final value. The system power supply should also change in a controlled manner, i.e., without excessive slew rate or changes in direction.
Power supplies must provide both static load current and dynamic capacitor charging current. A typical power supply has an output capacitor to stabilize an output voltage thereof and to provide a low output impedance at high frequencies greater than a unity gain bandwidth of the feedback systems of the power supply. The low output impedance minimizes disturbances on the output caused by high frequency pulsations of the load during normal operation. Power supplies also typically have additional load capacitors to further stabilize a voltage at a load.
A typical power supply has a practical limit of an amount of instantaneous power and current that the power supply can deliver to the load. The current and power limit is a determining factor in a size and cost of the power supply. If the limiting current capacity is exceeded by trying to force the power supply to change faster than the combined charging current of the output capacitor and the load current, the power supply will cease to be regulated by feedback systems, resulting in an occurrence of overshoots or undershoots and causing test errors in a production test system. Many loads are extremely sensitive to excessive voltage excursions, causing testing errors and possible device damage.
A typical application for system power supplies may need multiple different levels of voltage and current to provide power to a device under test (DUT). An auto-ranging power supply may be designed to provide more current at lower output voltages, and less current at higher output voltages, while minimizing the size of the power supply.
The auto-ranging power supply provides a different level of excess current depending on an instantaneous value of the output voltage. In order to provide a maximum rate of change of voltage for such a power supply, the slew rate of the power supply must change and decrease as the power supply voltage is increased, effectively reducing the level of output current required to slew any output capacitance connected to output terminals of the power supply which deliver the output voltage to the load as well as any internal output capacitance included in the design of the power supply.
In auto-ranging power supplies, an amount of excess current available to cause the output to change voltage is a function of the load current, the present output voltage and the current rating of the power supply at each operating point.
Switching power supplies must limit the maximum amount of current drawn to protect the components in the power supply from failure. Protection circuits are normally used to limit the maximum amount of current drawn by the power supply. If the protection circuits engage during normal output transitions, the protection circuits may cause overshoots or undershoots in the output when the protection circuits disengage. It is therefore desirable to cause the output of the power supply to increase during a programming increase at a rate that will draw the maximum amount of current that will not engage the protection circuits.
Auto-ranging power supplies, i.e., power supplies that provide an output current that increases as the output voltage decreases, require a dynamically varying level of current limiting as the output voltage increases in order to protect the switching components.
In order to rapidly reduce the voltage of a switching power supply, discharging the output capacitors of the power supply is necessary. The discharge of the output capacitors creates heat in at least one component within the power supply. A maximum discharge of the power supply is accomplished by maximizing the amount of heat generated within the limits of the available heat sinking capability of the down programming circuit at all output voltages. The operating locus that causes the maximum power at all operating points, and thus the minimum down programming time for an output discharge sequence is a constant power curve.
A system power supply is typically controlled by a Digital to Analog Converter (DAC) which is under computer control and has an output directly proportional to the output voltage up to a predetermined bandwidth limit.
Conventional circuits for optimizing programming speed of a power supply are well known. These devices include circuits for slew rate control, single stage RC low pass filter circuits, dual rate RC low pass filter circuits and overshoot control circuits.
In conventional slew rate control circuits, a constant slope is created on the programming voltage to the power supply. Slew rate control circuits have been implemented in a variety of ways, including digital and analog circuits. Although slew rate control circuits are relatively easy to implement, known implementations do not provide a optimum rate of change at all operating points.
In a conventional single stage RC low pass filter circuit, a single pole low pass filter is placed on a programming input signal to the power supply. Single stage RC low pass filter circuits are typically a lowest cost solution, however the single stage low pass filter circuit is independent of the absolute voltage and is determined solely by the change in voltage. Since a slope of the voltage change does not change with operating point, slower transitions to control the maximum slew rate to prevent overshoots from occurring are a result.
In a conventional dual rate RC low pass filter circuit, an absolute voltage and a relative voltage are used to switch in a slower time constant to try to optimize the slew rate. While the dual rate RC low pass filter circuit has superior output speed compared with the single stage RC low pass filter circuit, the dual rate RC low pass filter circuit is substantially more complex and costly, since the parts used are in the programming path and must be high precision to control accuracy of the voltage programming system.
In conventional overshoot control circuits, power and current limit circuits are used to determine a level of current and rate of change of the output voltage and current. Additional circuits are added that try to minimize or eliminate the overshoots that may occur when a main voltage control loop is out of regulation and control during transient conditions of overshoot. The overshoot control circuits are difficult to design for proper operation without causing instabilities and minimal overshoots over a wide set of output voltage and current operating points. The conventional overshoot control circuit has a benefit of providing a maximum slew rate, however the current or power limit circuit would need to be designed to provide a maximum amount of safe current at all operating points.